<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script type="text/javascript">
        var i;
        // alert(i); //undefined
        i = 12;
        //typeof ()是JS语言提供的一种函数
        // alert(typeof (i)); //number

        i = "abc";
        //它可以取变量的数据类型返回
        // alert(typeof (i)); //String

        var a = 12;
        var b = "abc";

        alert(a * b); //NaN是非数字，非数值
    </script>
</head>
<body>

</body>
</html>